1. Field of the Invention
The present invention relates to an optical fiber, and more particularly, to a optical fiber having spin.
2. Discussion of the Related Art
In recent years, long distance fiber optic communication has become increasingly important. In long distance optical fibers, it is important to retain the integrity of signals in the optical fibers. One problem in optical fiber signal integrity is Polarization Mode Dispersion (PMD).
PMD is the broadening of a optical signal pulse in a single mode fiber due to the dependence of the group velocity of the polarization state of the field, i.e., birefringence. In the case of constant birefringence, there are two states of polarization: one called "slow" and one called "fast". For any impulse at all, the superposition of the two states cause a temporary increase in pulse width which grows linearly with distance traveled. Therefore, the optical signal pulses will disperse, and the signals become unusable if the pulses combine.
In optical fibers, birefringence is caused by asymmetries and imperfections of the optical fibers such as ellipticity of the core and by anisotropies from internal stresses. A characteristic repeat length can be generally associated to these asymmetries and imperfections, being the length after which, on average, said asymmetries and imperfections are reproduced. Typical values for said repeat length are of the order of few meters to few hundred of meters. In addition, not only do fiber parameters vary, but also external stresses and geometric deformations are introduced by spooling, cabling, or installation.
These stresses create random couplings between the polarization modes of optical fibers. Further, the continuous exchange of power between the slow and the fast states limits the expansion of the impulse to a function related to the square root of the distance. Because these stresses are random, PMD is characterized by statistics. Typically a fiber's PMD is 0.05 to 0.5 ps km.sup.-1/2.
Further information on birefringence and on PMD in optical fibers can be found, e.g., in the following articles: W. Eickhoff et al. Applied Optics, Vol. 20, No. 19 pp. 3428-3435 (1981) and A. F. Judy International Wire & Cable Symposium Proceedings, pp. 658-664 (1994).
According to the above, it is desirable to reduce PMD as much as possible. There are two ways to reduce PMD: to reduce local birefringence and to increase the power exchange between the two polarization states.
To increase the power exchange, a method has been developed to apply a twist or a spin in the optical fiber. Twist refers to the rotation of a vitrified optical fiber about its axis whereas spin refers to rotation of a molten optical fiber. Both processes are similar with respect to their effects on the two polarization states. Furthermore, the both twist, or spin may be applied with turns constant in direction along the length of the optical fiber or with turns alternating in direction along the length of the optical fiber.
The amount of rotation applied to a twisted or spun fiber is characterized by the twist, .tau., which is defined by the number of turns per unit length. If twist is high, in relation to the previously mentioned repeat length, each of the two polarizations will be alternately in the slow and the fast states along fiber lengths shorter than the typical perturbation lengths. This results in a continuous and homogenous exchange of power between the two states, thereby significantly reducing the PMD.
Typically, spun fibers require .tau.=1-10 turns/m to induce a birefringence of .beta.=1-10 m.sup.-1 in order to overcome the effects of ellipticity and stress. When alternated twist is applied, the inversion period of the twist, i.e. the distance required to alternate the direction of the twist back and forth, is less critical and is typically 1-100 m.
The Applicant has afforded the problem of applying twist to the fiber in the molten phase, in order that such twist be frozen in the glass structure, when it is solidified in a cooling stage.
WO 83/00232, (Central Electricity Generating Board) discloses a method of making an optical fiber comprising drawing the fiber from a heated preform whilst effecting continuous relative rotation between the preform and the drawn fiber. To produce the spun fiber, the preform may be rotated during the drawing process.
The Applicant has observed that the method of rotating the preform requires the rotation of a large, potentially imbalanced, mass at high rotational velocity. For example, an optical fiber having an alternating twist of .tau.=1 turn/m and a draw speed of v.sub.draw =10 m/s requires the preform to rotate at 600 revolutions per minute. This can cause serious problems of vibrations in the fiber. As a result, the method is usually unsuitable.
U.S. Pat. No. 5,298,047, to Hart, Jr. et al., discloses that PMD can be substantially reduced if, during drawing of the fiber, a torque is applied to the fiber such that a "spin" is impressed on the fiber. Desirably the torque is applied such that the spin impressed on the fiber does not have a constant spatial frequency, e.g., has alternately clockwise and counterclockwise helicity. According to Hart, Jr. et al., the torque advantageously is applied at a point downstream from curing station, and it is most preferred to apply the torque by means of the first guide roller. The guide roller can be caused to oscillate back and forth or to move back and forth axially.
The above prior art methods require the rotation of a preform or the application of a torque, by means of a guide roller, while drawing the optical fiber.
The Applicant has observed that oscillation or movement of a guide roller requires a complex mechanical apparatus and can cause a relevant stress on the fiber coating just after its application and curing. Furthermore, conventional coating applications resist the transmission of torque from the roller to the uncooled optical fiber in the vicinity of the neckdown area of the furnace, thereby reducing effectiveness of the method.
Coating of glass optical fibers is desirable for the chemical and physical protection of the fibers. A common practice is to apply to the glass fiber a double-layer acrylic coating, whereby a first layer, with a relatively low elastic modulus, constitutes a "soft cushion" around the fiber and a further layer, with a relatively high elastic modulus, protects the fiber from the environment.
To give the fiber a homogeneous protection, it is important that each coating layer is concentric with the glass fiber. Concentricity of a layer, defined as the ratio between the minimum and maximum thickness of said layer in a section, is conveniently higher than 0.7, preferably higher than 0.85. Lower concentricity values correspond to a coating layer which is too thin on one side and, consequently, gives the fiber an insufficient protection. Increasing the coating layer thickness, while allowing low concentricity, might solve the protection problem, but would entail an increased bulkiness for the coated fiber and an increased cost.
To improve concentricity of the coating, the Applicant has tried use of a self-centering die, with a radial profile such as to cause high radial pressure, together with homogeneous feeding of the coating resin into the die, so as to get an homogeneous pressure. Both these measures have shown to be insufficient, as they can be made ineffective either by a slight lack of symmetry in the die or in the die holder, or by a slight lack of alignment of the drawing apparatus, both of which are difficult to avoid.